Source control is not like other systems. It's (largely) a backup of stuff that is stored elsewhere. Resilience/ monopoly concerns are much less.
I recently tried to get a small FOSS project to switch to Codeburg. The answer was "no" because the free CI for them let them catch some MacOS on Apple Silicon bugs (the devs don't have that hardware locally), and because they are already used to GitHub, making it easier to onboard people and review PRs.
If they remove some of that stuff, that would be worth complaining about. Until then, seems smart to take advantage of what GitHub is offering. Their CI isn't even super proprietary, you can basically just write bash scripts.
The phrase "pretty annoying/expensive to self host" highlights the source of those concerns.
How could one "just write bash scripts" to support macOS if you don't have Apple hardware, as in my example?
By then it’s too late, because there’s no alternative.